Patency of coronary artery bypass grafts is critical to continued patient benefit in those patients having coronary bypass surgery. The incidence of both early and late graft occulusion potentially negates the benefit of operation in up to 22 percent of patients. Evidence suggests that platelet mechanisms may be responsible for vein graft occlusion both early and late after surgery and that platelet inhibitor drugs may reduce the graft occlusion rate and the vein graft intimal proliferation. Other studies suggest that aortocoronary vein graft occlusion is particularly likely to occur in patients with shortened platelet survival time. Shortened platelet survival occurs in 50-60 percent of the patients with coronary artery disease and may be partially or completely corrected with platelet inhibitor drugs. In our preliminary studies of the platelet inhibitor drugs, dipyridamole and aspirin, we have demonstrated prolongation of platelet survival time in dogs as well as apparently beneficial alterations noted angiographically and histologically. A randomized, platelet inhibitor, drug trial with dipyridamole and aspirin versus nontreatment will be conducted in dogs. The objectives are to evaluate the effectivenesss of platelet inhibitor drugs in reducing the incidence of aortocoronary vein graft thrombotic occlusion that occurs in the first two weeks postoperatively and vein graft progressive intimal proliferation that occurs during the first postoperative year. Vein graft angiography with determination of patency and measurement of mean diameter will be conducted 2 weeks after surgery and again 6 months after surgery in all dogs. One-half of the treated and untreated dogs will bs sacrificed after the 6 month angiogram while the remaining half will be observed until 12 months after surgery. The latter dogs will have repeat angiography just prior to being sacrificed at 12 months. Pathologic evaluation of vein graft thrombosis and intimal proliferation will be made 6 and 12 months after surgery in dipyridamole and aspirin treated and untreated dogs. Platelet survival with 51Chromium labeled autologous platelets will be performed preoperatively and just prior to the sacrifice date in all dogs. Correlation of platelet survival time will be made with the angiographic and pathologic findings.